The present invention relates generally to methods and apparatus for treating or controlling medical, psychiatric or neurological disorders by application of modulating electrical signals to a selected nerve or nerve bundle of the patient, and more particularly to techniques for treating patients with neuropsychiatric disorders by application of such signals to the vagus nerve, using an implantable neurostimulating device. Specifically, the invention is directed toward treating the symptoms of neuropsychiatric disorders such as schizophrenia, depression, and borderline personality disorder, by selective modulation of vagus nerve activity.
Schizophrenia was initially thought to have only psychological origins. Advances in psychobiology and psychopharmacology have revealed that the illness is primarily organic in nature. Electrophysiologic studies of patients with schizophrenia have supported an organic etiology. Although not entirely consistent, electroencephalogram (EEG) studies have tended to reveal abnormalities in these patients. Also, some parallels have been found between schizophrenia and epilepsy.
In Psych. Res. ((1989) 29:419-420, Meuller reported finding increased beta (17.5 Hz) wave activity over the left central-temporal region during acute psychotic episode, whereas before and after the episode the frequency distribution in the EEG was normal. Williamson et al. in Can. J. Psych. (1989) 34:680-686, reported that a review of EEG mapping studies revealed that abnormalities exist, with some studies finding asymmetric fast activity while others reported primarily slowing. In Comprehensive Psych. (1990) 30(1):34-47, Keshaven et al. reported that sleep EEG studies in schizophrenic patients consistently showed abnormalities, and that although not specific to schizophrenia, patients tended to show impaired sleep continuity and reduced total sleep, but not all patients showed these abnormalities.
Gruzelier et al. reported in Int. J. Psychophysiol. (1990) 8:275-282, that in normal subjects the power of the beta II region of the EEG spectrum is decreased in cortical areas associated with specific mental tasks, this focal reduction in power being consistent with the thalamocortical EEG desynchronization response, and being decreased or absent in patients with schizophrenia. In Psychopathol. (1989) 22:65-140, Diehl indicated that acute psychotic episodes may be manifestations of temporal lobe epilepsy, and expressed the belief that disorders may exist in the ictal as well as the interictal phase. Kido et al. discussed six patients with seizures followed by schizophrenia-like states, in Japan J. Psych. Neurol. (1989) 43:433-438. In Intern. J. Neuroscience, Ardilla et al. described three cases in which patients diagnosed as psychotic were actually found to have complex partial status epilepticus.
Turning to depressive disorder, developments in psychobiology and psychopharmacology have provided considerable evidence that major depressive disorder and bipolar depression are biological rather than psychological diseases. Deficiency of brain neurostimulators has been associated with depression. In particular, abnormally low concentrations of serotonin and its metabolites have been found in depressed patients, as reviewed by Stark et al. in J. Clin. Psychopharmacol. (1985) 46[3, Sec.2]:7-13. Several serotonin uptake inhibitors, which increase the amount of serotonin at the synapse have been shown to be effective antidepressants. Serotonin is a neurotransmitter known to be involved in the brain stem projections of the vagus nerve in animals (Kilpatrick et al. in Eur. J. Pharmacol. (1989) 159:157-164) and in humans (Reynolds et al. Eur. J. Pharmacol. (1989) 174:127-130). It is postulated, then, that increased activity of the vagus nerve would be associated with release of more serotonin in the brain.
The conclusion that depression has a biological basis is also supported by numerous electrophysiological and endocrine studies.
A paper by Pollock et al. in Biol. Psychiatry (1990) 27:757-780, reported that a review of studies of the EEG in awake depressed patients reveals that alpha and beta activity are increased compared to controls. Elevations of delta and theta frequency ranges were possibly present as well. It was also felt that increased beta activity may be particularly prominent in patients with coexistent anxiety. Buysee et al. reported in Arch. Gen. Psych. (1988) 45:568-575, finding that sleep EEG of patients with primary depression and secondary dementia showed a higher percentage of rapid eye movement (REM) and more phasic REM activity and intensity than patients with primary dementia and secondary depression.
A strong relationship has been found to exist between sleep and depression. One of the most effective treatments for depression is sleep deprivation, which, however, is not a practical long term therapy. As with schizophrenia, a relationship also appears to exist between depression and seizures.
A substantial body of data suggests that anti-convulsant compounds have a spectrum of therapeutic efficacy in a variety of psychiatric syndromes which have not been associated with an epileptoid process. Pathological degrees of neuronal excitability and/or dysregulation may be associated with marked alterations in behavior, which are potentially treatable with anticonvulsant compounds, even in the absence of a concurrent seizure disorder.
The use of electroconvulsive therapy (ECT) to induce seizures is a primary treatment in acute depressive disturbances. ECT appears equal or superior to traditional psychopharmacological treatment modes with tricyclic antidepressants. Although the precise mechanism by which the effect of ECT is achieved is not fully known, it is thought to be related to biochemical changes in the brain resulting from synchronous discharges associated with seizures. Antidepressant drugs may produce similar changes but without inducing seizures.
Certain anticonvulsant agents such as carbamazepine are used in psychiatric disorders. Some studies have indicated dramatic improvement by carbamazepine in affective and schizophrenia-like syndromes associated with epilepsy. Non-epileptic patients with nonspecific EEG abnormalities who suffer from marked psychiatric disorders have also been shown to respond favorably to this drug. In this group, improvements in violent behavior, irritability, emotional lability, depression, agitation, and apathy have been reported. Anticonvulsant compounds thus appear to have an important spectrum of clinical activity in neuropsychiatric syndromes in addition to their clinical utility in the treatment of epileptic disorders.
Borderline personality disorder is a poorly understood, but recognized psychiatric disorder which seems to have some overlap of schizophrenia and depression. Patients tend to be poorly functional without florid psychosis or overt depression. Lahmeyer et al reported, in J. Clin. Psych. (1989) 50(6):217-225, that sleep architecture in patients with borderline personality disorder is disturbed in that REM latency is decreased and REM density is increased. This was found to be particularly true if patients suffered coexisting depression, a history of affective illness or a family history of psychopathology. Sleep abnormalities were reported to appear similar to those seen in affective disorders.
In addressing a therapy involving nerve stimulation to treat such neuropsychiatric disorders, observation should be made of existing knowledge that most nerves in the human body are composed of thousands of fibers, having different sizes designated by groups A, B and C, carrying signals to and from the brain and other parts of the body. The vagus nerve, for example, may have approximately 100,000 fibers (axons) of the three different types, each of which carries such signals. Each axon of that nerve only conducts in one direction, in normal circumstances. The A and B fibers are myelinated, that is, they have a myelin sheath in the form of a substance largely composed of fat. On the other hand, the C fibers are unmyelinated.
Myelinated fibers are typically larger, have faster electrical conduction and much lower electrical stimulation thresholds than the unmyelinated fibers. Along with the relatively small amounts of electrical energy needed to stimulate the myelinated fibers, it is noteworthy that such fibers exhibit a particular strength-duration curve in response to a specific width and amplitude of stimulation pulse.
The A and B fibers are stimulated with relatively narrow pulse widths, from 50 to 200 microseconds (.mu.s), for example. A fibers exhibit slightly faster electrical conductivities than the B fibers, and slightly lower electrical stimulation thresholds. The C fibers are relatively much smaller, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring wider pulse widths (e.g., 300-1000 .mu.s) and higher amplitudes for activation. Although the A and B fibers may be selectively stimulated without also stimulating the C fibers, the magnitude and width of the pulse required for stimulating the C fibers would also activate A and B fibers.
Although electrical stimulation of the nerve fiber typically activates neural signals in both directions (bidirectionally), selective unidirectional stimulation is achievable through the use of special nerve electrodes and stimulating waveforms. As noted above, each axon of the vagus nerve normally conducts in only one direction.
In a paper on the effects of vagal stimulation on experimentally induced seizures in rats (Epilepsia 1990, 31 (Supp 2): S7-S19), Woodbury has noted that the vagus nerve is composed of somatic and visceral afferents (i.e., inward conducting nerve fibers which convey impulses toward a nerve center such as the brain or spinal cord) and efferents (i.e., outward conducting nerve fibers which convey impulses to an effector to stimulate it and produce activity). The vast majority of vagal nerve fibers are C fibers, and a majority are visceral afferents having cell bodies lying in masses or ganglia in the neck. The central projections terminate, by and large, in the nucleus of the solitary tract which sends fibers to various regions of the brain (e.g, the hypothalamus, thalamus, and amygdala); others continue to the medial reticular formation of the medulla, the cerebellum, the nucleus cuneatus and other regions.
Woodbury further notes that stimulation of vagal nerve afferent fibers in animals evokes detectable changes of the EEG in all of these regions, and that the nature and extent of these EEG changes depends on the stimulation parameters. Chase, in Exp Neurol (1966) 16:36-49, had also observed that vagal activation can affect the EEG activity of certain parts of the brain. The applicants herein postulate that synchronization of the EEG may be produced when high frequency (&gt;70 Hz) weak stimuli activate only the myelinated (A and B) nerve fibers, and that desynchronization of the EEG occurs when intensity of the stimulus is increased to a level that activates the unmyelinated (C) nerve fibers. Woodbury also observes that vagal stimulation can produce widespread inhibitory effects on seizures and certain involuntary movements.
Extra-physiologic electrical stimulation of the vagus nerve has previously been proposed for treatment of epilepsy and various forms of involuntary movement disorders. Specifically, in U.S. Pat. No. 4,702,254 issued Oct. 27, 1987 to J. Zabara (referred to herein as "the '254 patent"), a method and implantable device are disclosed for alleviating or preventing epileptic seizures, characterized by abnormal neural discharge patterns of the brain. The '254 patent describes an implantable neurocybernetic prosthesis (NCP) which utilizes neurocybernetic spectral discrimination by tuning the external current of the NCP generator to the electrochemical properties of a specific group of inhibitory nerves that affect the reticular system of the brain. These nerves are embedded within a bundle of other nerves, and are selectively activated directly or indirectly by the tuning of the NCP to augment states of brain neural discharge to control convulsions or seizures. According to the patent, the spectral discrimination analysis dictates that certain electrical parameters of the NCP pulse generator be selected based on the electrochemical properties of the nerves desired to be activated. The patent further indicates that the optimum sites for application of the NCP generator output to produce the desired effects are the cranial nerves in general, and the vagus nerve in particular.
The NCP disclosed in the '254 patent may be activated either manually or automatically, to provide treatment for the duration of the seizure. Manual activation is performed when the patient experiences the aura at onset of the seizure. Alternatively, automatic activation may be triggered upon detection of instantaneous changes in certain state parameters immediately preceding or at onset of a seizure. Additionally, a prophylactic or preventive mode may be employed in which the NCP is activated periodically to reduce the occurrence and/or the intensity of the seizures. The NCP stimulator of the '254 patent is implanted in the patient's chest and is connected to electrodes installed at the selected point of signal application at the nerve site with the more negative electrode situated closer to the brain and the positive electrode further from the brain, along the vagus nerve.
It is a principal object of the present invention to apply the techniques of selective modulation of vagus nerve electrical activity, using a neurostimulator device which may be implantable, or used external to the body with only a small portion of the circuitry implanted or with only the nerve electrode(s) and associated lead(s) implanted percutaneously in the body, to the treatment of neuropsychiatric disorders including schizophrenia, depression, and borderline personality disorder.